How to Identify the difference between distributor caps

I get asked all the time how come my distributor cap will not fit. There are two types one for the common aluminum case distributor that fit both point and electronic units then there is the dualpoint cap that fits only the cast iron distributor housings. The aluminum has a internal slot that the tab on the vacuum advance fits into and the dual point (prestolight) distributor has a tab on the cap that fits into a slot on the hold down clip. Here are some pictures of the difference.